I added a certification starting September 2018 to my developer story and marked the certification as current. The date math, however, shows eleven months:

I can't think of a way the date math through July 10, 2019, would legitimately compute as greater than ten months. September 1, 2018, through July 31, 2019, could be eleven months, though waiting until August 1 seems simpler.

If it ignores the days and just count months, you get September, October... June, July = 11 months. That's effectively the same thing as September 1, 2018, through July 31, 2019, but a much more basic program counting months instead of days.
– Davy MJul 10 at 18:52

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@DavyM Fair point. I think the linked questions have gone through this logic some. Arguably, though, if one intends to compute a duration, one point to the same point makes more sense as zero. For example, September 2018 - September 2018 seems in a way like it should be zero, not one, but I could see the opposite argument as well. Sept.-Oct. 2018, on the other hand, seems more like one month to me instead of two, which is the logic I applied in my post. In summary, date math is bad.
– AndrewJul 10 at 20:07

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I think this is an application of "common sense" that would be appreciated by a hiring manager rather than "accuracy" that would be appreciated by a dev. If you worked at a place from September to July the guy reading your resumé would be unsurprised to see it rounded up to 11 months in the absence of a date saying 30th sep to 1st July. It's a win for you because it overviews your experience as longer than it is, and while it might not appeal to the mathematician in you I think it will do you a favor while appealing to the target audience
– Caius JardJul 11 at 5:54